A motor can be combined with a compressor in a single housing to provide a motor-compressor system. Generally, the motor resides in one cavity or compartment of the housing, while the compressor resides in a separate cavity or compartment. The motor drives the compressor, typically using a shared shaft, or with two or more shafts coupled together, in order to generate a flow of compressed process gas. In hermetically sealed units, the shaft is typically supported by two or more magnetic journal bearings and often includes additional magnetic bearings for thrust compensation.
Magnetic bearings and the electric motor are susceptible to damage if they come into contact with unfiltered or “dirty” process gas (i.e., the gas being compressed by the compressor). Such process gas can include any number of damaging materials, such as dirt, metal, oil, water, particulate matter, or the like. To avoid the motor and bearings coming into contact with dirty process gas, shaft seals are installed between the compressor and the bearings. These seals are typically fed with seal gas, such as filtered process gas, at a pressure slightly higher than the pressure within the compressor. The seal gas thus precludes dirty process gas from leaking into and past the seals.
Seal gas is often made up of gas taken from the discharge of the compressor. Accordingly, if the compressor does not provide sufficient process gas at the required pressure to feed the seals, the seals may become ineffective, allowing dirty process gas to leak and come into contact with the motor and bearings. One example of when this can occur is during settle out after a shutdown, in which the process side reaches a pressure level that is higher than the seal gas injection pressure. Unless the pressure differential across the seals is rapidly reversed, this dirty process gas may contact the bearings and/or the motor, potentially damaging one or both of these components. Furthermore, a lack of seal gas pressure may result in a large pressure differential across the seals, which can damage the seals themselves.
What is needed is an efficient system and method for rapidly pressurizing the motor compartment and bearings to keep the dirty process gas from contacting the motor and bearings in situations where the seal gas becomes insufficient.